Mass transfer apparatus can take the form of a membrane oxygenator, which is used to oxygenate blood. Oxygenation of blood is important, for example, in performing surgical procedures, such as open heart surgery, where the heart is stopped and the patient's blood is pumped artificially, requiring oxygenation. One type of conventional membrane oxygenator employs bundles of stationary hollow fibers retained within a cylindrical housing wherein oxygen is pumped through the hollow fibers in the same direction as the blood being pumped through the oxygenator housing. The hollow fibers consist of microporous membranes, which are impermeable to blood and permeable to gas. Gas exchange takes place when venous blood flows through the housing and contacts the hollow fibers. Based on the law of diffusion, oxygen diffuses across the hollow fiber walls and enriches venous blood in contact with these hollow fibers. Examples of this type of membrane oxygenator are described in U.S. Pat. No. 5 4,620,965 issued to Fukusawa et al. and U.S. Pat. No. 4,698,207 issued to Bringham et al. The disadvantage of this type of membrane oxygenator is that a relatively thick blood boundary layer is formed around the hollow fibers, which retards oxygenation of blood that does not directly contact the hollow fibers.
In order to disrupt the blood boundary layer, another type of conventional membrane oxygenator oxygenates blood by directing blood flow substantially perpendicular or at an angle to the hollow membranes carrying the oxygen. Examples of this type of membrane oxygenator are described in U.S. Pat. No. 4,639,353 issued to Takemura et al., U.S. Pat. No. 3,998,593 issued to Yoshida et al. and U.S. Pat. No. 4,490,331 issued to Steg, Jr. Drawbacks to these designs include the need for a large priming volume and large blood-biomaterial exposure area, and the tendency for the permeability of the hollow membranes to decrease over time, causing the oxygenator to become less efficient.
Yet another type of membrane oxygenator discloses moving a part of the oxygenator in order to provide increased mixing of blood and oxygen. Examples of this 20 type of membrane oxygenator are described in U.S. Pat. Nos. 3,674,440 and 3,841,837 issued to Kitrilakis and Kitrilakis et al., collectively, (the “Kitrilakis Patents”) and U.S. Pat. No. 3,026,871 issued to Thomas (the “Thomas Patent”). The Kitrilakis Patents disclose a blood flow path positioned around a rotor, wherein the blood flow path and the oxygen flow path is separated by a non-porous membrane layer through 25 which the blood cannot flow. The blood flow travels substantially parallel to the oxygen flow and rotation of the rotor causing mixing by a shearing effect. A characteristic of this device is the use of structures with a wafer-like membrane to separate the blood from the gas phase. In contrast, a distinguishing characteristic of the current device described herein is the use of hollow fiber membranes that both improve pumping action 30 and significantly increase the amount of surface area available for mass transfer. Although the Kitrilakis oxygenator may provide a degree of mixing of the blood, this type of mixing may lead to destruction of red blood cells. While hollow fiber membranes have been and are currently used to oxygenate blood, the devices in which they are used require a separate blood pump, and existing adult units require approximately 2–3 rn2 of surface area from the fibers. In contrast, the present invention requires no separate pump, and as little as 0.5 m2 of surface area from the hollow fiber membranes.
The Thomas Patent discloses rotating a single, cylindrical, semi-permeable membrane containing oxygen in a housing wherein blood contacts and flows over the membrane and oxygenation of the blood occurs across the rotating membrane. Disadvantages of this type of membrane oxygenator are that it too tends to form a blood boundary layer along the surface of the membrane. The diffusion of oxygen and carbon dioxide through this blood boundary layer is poor due to the thickness of the boundary layer. Furthermore, since blood films form along the surface of the membrane cylinder there is no mechanism for creating a cross flow component to disrupt the static boundary layer. Accordingly, the overall oxygen and carbon dioxide transfer of this device is poor and the device requires large priming volumes in order to be properly operated.
Yet another type of blood oxygenator device comprises short microporous fiber layers which are folded, twisted and woven around a hollow shaft that carries the inlet and outlet gas flows. The device is implanted into the vascular system of a patient and rotated to cause mixing of the blood. This type of device is explained in greater detail in “A Dynamic Intravascular Lung,” ASAIO Journal. 1994. A disadvantage of this type of blood oxygenator is that only limited number of fiber layers can be incorporated into the device. This restriction occurs because anatomical space is limited and results in insufficient oxygenation/decarbonation of blood. Furthermore, the rotation of the device within the blood vessel may destroy the cells lining the blood vessel.
With the exception of the Kitrilakis device, all of the blood oxygenators mentioned above require a separate pump apparatus to propel the blood through the oxygenator. Some blood oxygenators even employ two separate pump apparatus, herein a venous pump is used to pump venous blood to the oxygenator and an arterial pump is used to pump the oxygenated blood from the oxygenator to the patient's arteries. Examples of this type of pump-oxygenator system are disclosed in U.S. Pat. Nos. 3,907,504 and 3,927,980 issued to Hammond et al. and Leonard, respectively. The major disadvantage of pump-oxygenators that employ a multiple step process to pump and oxygenate the blood is that blood may be damaged. Also, this type of approach requires considerable fluid volume to prime the pumps, which leads to clinical complications, difficulty in patient management, and a bulky construction.
Nowhere in the cited related art is there disclosed a combined mass transfer and pump apparatus which effectively oxygenates and pumps blood in one step to sustain a patient for an extended duration, wherein the apparatus is a compact unit. Therefore, there is a definite need for this combination pump-oxygenator, which provides for effective oxygenation/decarbonation and pumping of blood as disclosed in the present invention.